In electrostatography, image charge patterns are formed on a support and are developed by treatment with an electrostatographic developer containing marking particles which are attracted to the charge patterns. These particles are called toner particles or, collectively, toner. The image charge pattern, also referred to as an electrostatic latent image, is formed on an insulative surface of an electrostatographic element by any of a variety of methods. For example, the electrostatic latent image may be formed electrophotographically, by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate. Alternatively, the electrostatic latent image may be formed by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material.
One well-known type of electrostatographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in cascade and magnetic brush electrostatographic development processes. The toner particles and carrier particles differ triboelectrically, such that during mixing to form the developer, the toner particles acquire a charge of one polarity and the carrier particles acquire a charge of the opposite polarity. The opposite charges cause the toner particles to cling to the carrier particles. During development, the electrostatic forces of the latent image, sometimes in combination with an additional applied field, attract the toner particles. The toner particles are pulled away from the carrier particles and become electrostatically attached, in imagewise relation, to the latent image bearing surface. The resultant toner image can then be fixed, by application of heat or other known methods, depending upon the nature of the toner image and the surface, or can be transferred to another surface and then fixed.
A number of requirements are implicit in such development schemes. Namely, the electrostatic attraction between the toner and carrier particles must be strong enough to keep the toner particles held to the surfaces of the carrier particles while the developer is being transported to and brought into contact with the latent image, but when that contact occurs, the electrostatic attraction between the toner particles and the latent image must be even stronger, so that the toner particles are thereby pulled away from the carrier particles and deposited on the latent image-bearing surface.
Carrier particles comprise a core usually coated with a polymer. Commonly used polymers include: silicone resin; acrylic polymers, such as, poly(methylmethacrylate); and vinyl polymers, such as polystyrene and combinations of materials. Another commonly used coating material is fluorohydrocarbon polymer, such as poly(vinylidene fluoride) or poly(vinylidene fluoride-co-tetrafluoroethylene). See, for example, U.S. Pat. Nos. 4,546,060; 4,478,925; 4,076,857; and 3,970,571. Such polymeric fluorohydrocarbon carrier coatings can serve a number of known purposes. One such purpose can be to aid the developer to meet the electrostatic force requirements mentioned above by shifting the carrier particles to a position in the triboelectric series different from that of the uncoated carrier core material, in order to adjust the degree of triboelectric charging of both the carrier and the toner particles. Another purpose can be to reduce the frictional characteristics of the carrier particles in order to improve developer flow properties. Still another purpose can be to reduce the surface hardness of the carrier particles so that they are less likely to break apart during use and less likely to abrade surfaces, such as photoconductive element surfaces, that they contact during use. Yet another purpose can be to reduce the tendency of toner material or other developer additives to become undesirably permanently adhered to carrier surfaces during developer use (often referred to as "scumming"). A further purpose can be to alter the electrical resistance of the carrier particles. All of these, and even more, purposes are well known in the art for polymeric fluorohydrocarbon carrier coatings. What is desired is a carrier coating that serves all of the above-noted purposes well, both initially and throughout a long useful life involving many replenishments of toner.
Additional problems are faced by carriers used in electrophotographic equipment in which there is a great deal of developer mixing, sometimes referred to as "exercising". Examples of such equipment is disclosed in U.S. Pat. Nos. 4,878,089 and 4,714,046. In such uses, loss of carrier charge with exercising or aging is one of the biggest problem encountered. This drop in charge with life leads to many problems which include unacceptable dusting, poor image quality and reduced reliability of the equipment. Among various techniques practiced to address this shortcoming, preconditioning of the developer is by far the most common. In preconditioning, carrier is mixed with toner, exercised and stripped before use in the electrophotographic copier or printer. This initial "aging" or treatment of the developer outside the equipment is expensive and labor intensive and only addresses the problem of initial charge instability. The problem that is not addressed is carrier aging that occurs when the developer is exercised extensively when a very low amount of toner is being developed or placed on each copy.
Various methods have been used to improve the characteristics of fluorocarbon coated carriers. For example, U.S. Pat. No. 4,737,435 to Yoerger, disclosed a method of dehydrofluorinating a fluorohydrocarbon carrier coating by contacting the coated carrier particles with a basic solution. The resulting change in chemical structure had the effect of repositioning the carrier triboelectrically. In another example, U.S. Pat. No. 4,726,994 to Yoerger, a method is disclosed for dehydrofluorinating and oxidizing a fluorohydrocarbon carrier coating by contacting the coated carrier particles with a basic solution and with an oxidizing agent. The resulting change in chemical structure also had the effect of repositioning the carrier triboelectrically, and in addition, decreased overcharging.
Polyvinylidene Fluoride (PVF.sub.2) is commonly used as a carrier coating, because it has a position in a triboelectric series with a variety of electrophotographic toners that helps insure that a positive charge is imparted to the toner and a negative charge to the carrier surface. PVF.sub.2 can exist in a non-polar .alpha.-phase, and polar .beta., and .gamma. phases. PVF.sub.2 is normally applied to carrier cores by melt coating at a temperature in the range of about 190.degree. C. to 260.degree. C. PVF.sub.2 solidifies from a melt as .alpha.-phase. Melt coating is highly preferred over solvent coating for environmental reasons. M. Kobayashi, K. Tashiro and H. Tadokoro, Macromolecules., Vol. 8, p. 158 (1975) teaches the preparation of .alpha.-phase PVF.sub.2 by stretching melt-crystallized specimens or by rolling film speciments cast from dimethylacetamide solution. W. M. Priest, Jr., and D. J. Luca., J. Appl. Phys., Vol. 46, p. 4136 (1975) teaches the preparation of .beta.-phase PVF.sub.2 by poling. W. W. Doll and J. B. Lando, J. Macromol. Sci.-Phys., Vol. B2, p. 219 (1968) teaches the preparation of .beta.-phase PVF.sub.2 by high pressure crystallization. M. A. Marcus, paper presented at the Fifth International Meeting on Ferroelectricity at Pennsylvenia Sate University, Aug. 17-21, (1981) teaches the preparation of .beta.-phase PVF.sub.2 by plastic deformation under high pressure. Such procedures are expensive and complex.
There is thus a continuing need for a carrier that has good electrophotographic properties, both initially and throughout a long useful life involving many replenishments of toner and for developer incorporating that carrier. There is also a continuing need for a simple, inexpensive method for converting .alpha.-PVF.sub.2 to .beta.-PVF .sub.2.